It was recognized by early workers that coal can be liquified by controlled heating in the substantial absence of oxygen. The conversion products are a liquid and a char. Because of the new compelling economic factors, the technology of coal liquefaction and gasification has been expanding at an accelerated pace. Pioneer developments in the field are represented by Lurgi and Fischer-Tropsch technology. More recent advances in coal liquefaction are described in U.S. Pat. Nos. 1,904,586; 1,955,041; 1,996,009; 2,091,354; 2,174,184; 2,714,086; 3,375,188; 3,379,638; 3,607,718; 3,640,816 3,642,608; 3,705,092; 3,849,287; 3,870,621; inter alia.
Hence, by recently developed methods coal can be converted into hydrocarbonaceous liquids by subjecting coal to reaction with solvents, with or without hydrogen, to yield a slurry mixture of dissolved coal and a residuum of unreacted coal and mineral ash. The solids are present in the slurry as a finely divided suspension of insoluble matter.
The suspended solids have a high content of ash and sulfur which reduce the value of the coal as a fuel and for specialized applications. For this reason, it is normal procedure in coal liquefaction operations to separate the undesirable finely divided solids prior to further processing of coal liquefaction products.
The procedure for separating the solids from the coal liquefaction solvent phase has been a serious economic disadvantage in coal liquefaction processes. The fine particle size characteristic of the suspended solids creates formidable difficulties in the application of conventional separation techniques such as filtration, centrifugation and settling.
Existing filtration techniques are unattractive because of slow filtration rates and plugging of filters.
Gravity settling techniques have limited application because of slow settling rates and inefficient ash removal.
Centrifugation techniques for removing the suspended solids have the disadvantage of high equipment and operating costs.
Because of the deficiencies of conventional separation techniques, there remains a need for an effective method for separating finely divided solids from the solvent-rich extract phase of coal liquefaction processes.
Accordingly, it is an object of the present invention to provide an improved coal liquefaction process.
It is another object of this invention to provide an economic method for separating the finely divided suspension of residual solids from the solvent-rich extract phase of a coal liquefaction product.
Other objects and advantages shall become apparent from the following description and examples.